The present invention relates to a polyoxymethylene resin modifier for prominently improving mechanical properties of the molded articles, and a polyoxymethylene resin composition containing such a ye modifier. The polyoxymethylene resin composition of the present invention is capable of easily providing molded articles having excellent mechanical properties, especially flexural modulus, Rockwell hardness and weldability as well as high chemical resistance and good visual appearance, so that it finds its way into many uses, such as parts of OA equipment, electrical and electronic devices, automobiles, etc.
Polyoxymethylene resin offers a good balance of mechanical properties, fatigue resistance, wear resistance, chemical resistance and moldability, and is widely utilized for molding parts of various industrial products such as automobiles, electrical and electronic devices, precision machines, building materials, piping, etc. On the other hand, there have been proposed the polyoxymethylene resin compositions in which reinforcements such as glass fiber, carbon fiber, alumina fiber, glass flakes, talc, etc., are incorporated for further improving mechanical properties (JP-A-62-91551, U.S. Pat. No. 4,806,586, etc.). However, reinforced oxymethylene polymer resins containing reinforcements such as mentioned above, as compared with non-reinforced oxymethylene polymer resins, are excessively lowered in fluidity when melted, so that when they are molded under high temperature and high injection pressure conditions, the molded articles tend to have poor appearance due to heat deterioration or discoloration of the resin. Further, orientation of the reinforcement gives rise to such problems as reduced weldability and warping of the molded articles.
It has been also proposed to add an inorganic crystal nucleating agent to oxymethylene polymers for improving mechanical properties (e. g. JP-A-47-11136). In this composition, however, the degree of improvement of mechanical properties is small in comparison with the compositions to which reinforcements were added, and the property requirements are not necessarily satisfied.
As a result of extensive studies for overcoming the above problems, the present inventors found that a polyoxymethylene resin composition obtained by incorporating a straight-chain low-molecular-weight oxymethylene polymer (A) having an unprecedentedly sharp molecular weight distribution (Mw/Mn) that falls within the range of 1.0 to 3.0 while having a number average molecular weight in the range of 1,000 to 8,000 is possessed of various excellent properties at the same time, enabling easy production of the molded articles having high-grade mechanical properties, especially flexural modulus, Rockwell hardness and weldability as well as high chemical resistance and good visual appearance.
Proposals relating to the preparation of low-molecular-weight oxymethylene polymers have been made in some patents, for example, JP-B-55-39182, JP-A-6-172612 and EP-A-708123. In these patents, however, no method is disclosed in which molecular weight distribution (Mw/Mn) is kept under 3.0. In these proposals, a large quantity of chain transfer agent is used for obtaining a desired low-molecular-weight oxymethylene polymer, and it is supposed that in order to compensate for the consequent drop of polymerizability, a batch polymerizer or a large amount of polymerization catalyst was used. According to these methods, it is impossible to obtain the polymers having Mw/Mn of not greater than 3.0, and the obtained polymers are incapable of producing the above-said outstanding effects of the present invention, such as high-grade mechanical properties, high chemical resistance and good visual appearance of the molded product.
JP-B-45-35188 discloses a method for obtaining a low-molecular-weight oxymethylene polymer by starting with paraformaldehyde. This method is also unable to produce the said effects since the molecular weight distribution (Mw/Mn) exceeds 3.0.
JP-A-2-55712 and U.S. Pat. No. 5,248,762 disclose cyclic oxymethylene polymers having a molecular weight distribution (Mw/Mn) in the range of 1.0 to 1.5 while having a number-average molecular weight in the range of 500 to 5,000. These polymers also fall short of providing the above-said effects as they have no straight-chain molecular skeleton of the polymer of the present invention.
The present invention is designed to solve the prior art problems mentioned above, and has for its object to provide a polymer resin composition which is remarkably improved in mechanical properties, especially flexural modulus, without using reinforcements, also shows good fluidity and high thermal stability when melted, and hence is capable of easily producing the molded articles with good visual appearance with no fear of impairing their weldability and anti-warping properties.
As a result of many studies conducted to solve the above problems, the present inventors found that a polyoxymethylene resin composition obtained by uniformly mixing (A) 1 to 500 parts by weight of a straight-chain low-molecular-weight oxymethylene polymer characterized by containing 0 to 30 mol % of a co-monomer having a number-average molecular weight of 1,000 to 8,000 and a molecular weight distribution (Mw/Mn) of 1.0 to 3.0 with (B) 100 parts by weight of a high-molecular-weight polyoxymethylene resin having a number-average molecular weight of 15,000 to 1,000,000 is greatly improved in mechanical properties, especially flexural modulus, without using reinforcements, also has good fluidity and thermal stability in the molten form, hence is long in critical residence time till generation of silver streaking and thus capable of easily producing the molded articles with excellent visual appearance without impairing their weldability and anti-warping properties, and on top of that, has a surprising effect of improving Rockwell hardness and chemical resistance of the molded articles.
Thus, the present invention provides:
(1) A low-molecular-weight oxymethylene polymer having a straight-chain molecular structure, a number-average molecular weight of 1,000 to 8,000 and a molecular weight distribution (Mw/Mn) of 1.0 to 3.0, and containing 0 to 30 mol % of a co-monomer unit.
(2) A low-molecular-weight oxymethylene polymer described in (1) above, wherein the content of the co-monomer unit is 0 to 0.8 mol %.
(3) A low-molecular-weight oxymethylene polymer described in (1) or (2) above, wherein the content of the comonomer unit and the melting point of the polymer fall in the region defined by DEFG of FIG. 1.
(4) A low-molecular-weight oxymethylene polymer described in any one of (1) to (3) produced by continuous bulk polymerization reaction using as starting materials a monomer or a mixture of a monomer and a co-monomer, and a molecular weight modifier in an amount of 0.1 to 1 mole per 1 kg of said monomer or said mixture of a monomer and a co-monomer, and also using a cationic polymerization initiator as polymerization catalyst in an amount of 0.00001 to 0.0001 mole per 1 mole of said starting materials.
(5) A polyoxymethylene resin composition comprising (A) 1 to 500 parts by weight of a low-molecular-weight oxymethylene polymer described in any one of (1) to (4) above and (B) 100 parts by weight of a high-molecular weight polyoxymethylene resin having a number-average molecular weight of 15,000 to 1,000,000 and selected from the group consisting of (B-1) oxymethylene homopolymer resin, (B-2) oxymethylene copolymer resin, (B-3) oxymethylene block copolymer resin and a mixture thereof.
(6) A polyoxymethylene resin composition described in (5) above wherein the content of the low-molecular-weight oxymethylene polymer (A) is 10 to 100 parts by weight.
(7) A polyoxymethylene resin composition described in (5) or (6) above wherein the high-molecular-weight polyoxymethylene resin (B) is an oxymethylene copolymer resin (B-2) having a melting point of 167 to 171xc2x0 C.
(8) A polyoxymethylene resin composition described in any one of (5) to (7) above wherein the polyoxymethylene resin composition contains 1 to 10,000 ppm of a crystal nucleating agent.
(9) A process for continuously producing a polyoxymethylene resin composition described in any one of (5) to (8) above, which comprises the steps of producing a crude polymer of the low-molecular-weight oxymethylene polymer (A) and a crude polymer of the high-molecular-weight polyoxymethylene resin (B) simultaneously by the separate polymerizers by using equipment consisting of at least two continuous bulk polymerizers connected in parallel and an extruder disposed successively thereto, blending 100 parts by weight of said crude polymer of the high-molecular-weight polyoxymethylene resin (B) and 1 to 500 parts by weight of said crude polymer of the low-molecular-weight oxymethylene polymer (A) while deactivating the polymerization catalyst, and continuously and uniformly melting and mixing said crude polymers while conducting a terminal stabilization treatment on said crude polymers by said extruder.
(10) A process for continuously producing a polyoxymethylene resin composition described in any one of (5) to (8) above, which comprises the steps of polymerizing a monomer or a mixture of a monomer and a co-monomer and a molecular weight modifier with a cationic polymerization initiator as polymerization catalyst by using one or two or more continuous bulk polymerizers connected in series, and further carrying out polymerization by adding to the resulting polymerization product said monomer or said mixture of a monomer and a comonomer and said molecular weight modifier in a state not causing deactivation of said cationic initiator so that the amount of the obtained low-molecular-weight oxymethylene polymer will become 1 to 500 parts by weight per 100 parts by weight of the high-molecular-weight polyoxymethylene resin.
(11) A process for continuously producing a polyoxymethylene resin composition described in any one of (5) to (8) above, which comprises the steps of producing a crude polymer of the low-molecular-weight oxymethylene polymer (A) and a crude polymer of the high-molecular-weight polyoxymethylene resin (B) simultaneously by the separate polymerizers using formaldehyde and a molecular weight modifier as starting materials and also using an onium salt-based polymerization catalyst by using a series of equipment consisting of at least two slurry polymerizers connected in series or parallel, a terminal stabilizing device and an extruder disposed successively, blending 100 parts by weight of said crude polymer of the high-molecular-weight polyoxymethylene resin (B) and 1 to 500 parts by weight of said crude polymer of the low-molecular weight oxymethylene polymer (A) while deactivating and removing the polymerization catalyst, blocking the unstable terminals of the obtained polymer, and continuously conducting uniform melting and mixing by the extruder.
(12) Parts of OA equipment, music, video and information devices, communications apparatus, electrical and electronic devices, toys, sports equipment, furniture, housing equipment or automobiles, obtained by molding a polyoxymethylene resin composition described in any one of (5) to (8) above.